Having descended from the first multicellular animals on earth, sponges are a key group in which to seek
innovations that form the basis of the metazoan body plan, but sponges themselves have a body plan that is extremely
difficult to reconcile with that of other animals. Adult sponges lack overt anterior–posterior polarity and sensory organs,
and whether they possess true tissues is even debated. Nevertheless, sexual reproduction occurs as in other metazoans,
with the development of embryos through a structured series of cellular divisions and organized rearrangements
of cellular material, using both mesenchymal and epithelial movements to form a multicellular embryo. In most cases,
the embryo undergoes morphogenesis into a spatially organized larva that has several cell layers, anterior–posterior polarity,
and sensory capabilities. Here we review original data on the mode of cleavage, timing of cellular differentiation,
and the mechanisms involved in the organization of differentiated cells to form the highly structured sponge larva. Our
ultimate goal is to develop interpretations of the phylogenetic importance of these data within the Porifera and among
basal Metazoa.